1. Field of the Invention
The present invention relates to a workpiece blanking apparatus for blanking a workpiece of predetermined shape out of a metal plate.
2. Description of the Related Art
Known workpiece blanking apparatus comprise a vertically movable upper die assembly, a blanking punch vertically movable with respect to the upper die assembly and having a shape corresponding to the shape of a workpiece to be blanked, and a lower die assembly with a die fixedly mounted thereon. When the blanking punch is lowered toward a metal plate placed on the die, a desired workpiece is blanked out of the metal plate.
Specifically, a pad biased to move downwardly by a biasing means is disposed around the blanking punch. The pad has a lower end which is at the same height as the lower end of the blanking punch or projects slightly downwardly from the lower end of the blanking punch when the upper die assembly is in an elevated position. When the upper die assembly is lowered, the pad descends with the blanking punch and engages the metal plate on the die simultaneously with or slightly earlier than the blanking punch, thus resiliently holding the metal plate on the die. The blanking punch is continuously lowered into the metal plate to blank the workpiece out of the metal plate and depress the blanked workpiece.
The die on the lower die assembly has a vertically movable counterpunch disposed in confronting relationship to the blanking punch and normally biased to move upwardly. The counterpunch has an upper end which lies at the same height as the die when the upper die assembly is in the elevated position, i.e., before the workpiece is blanked out of the metal plate. As the workpiece is depressed by the blanking punch, the counterpunch is lowered while being pressed against the lower surface of the workpiece. After the workpiece is blanked, the blanking punch is lifted to a predetermined position, e.g., a position above the die, the pad is released from the downward push and displaced upwardly away from the metal plate.
Since the counterpunch is biased upwardly, when the blanking punch is lifted, the workpiece is raised to the position of the upper surface of the die by the counterpunch. Therefore, before the pad is spaced from the metal plate, the workpiece is pushed back to the metal plate, and tends to be deformed because its outer edge contacts the severed edge of the metal plate. As a result, the blanked workpiece is liable to have an inaccurate shape.